Our studies of gene regulation relating to malignancy included primate retrovirus expression, and modulation of human chorionic gonadotropin, alkaline phosphatase, and type IV collagen. The effort to induce new primate retroviral expression centers around three areas; (1) use of cell fusion and chromosome segregation; (2) transfection by the early region of SV 40 DNA ("Gene A") of unusual cell strains to obtain permanent indicator lines to overcome host range restriction and for use in retroviral induction; and (3) use of Rhesus monkey and human trophoblast in cell and organ culture as targets for retroviral isolation. Improved methods have been developed for growth of human placenta in organ culture and for isolation of trophoblast cells for growth in monolayer culture. First-trimester Rhesus placental tissue, but not Rhesus fetal lung, kidney, heart, spleen, intestine, or brain, contains MAC-1 p26 expression. This endogenous type C particle polypeptide can also be detected in Rhesus placenta in organ culture. Rhesus trophoblasts transfected by SV 40 Gene A display prolonged growth in cell culture, and expression of alkaline phosphatase activity, type IV collagen, fibronectin and by EM contain desmosomes.